AQA alevel astrophysics

describe the Doppler effect

the compression or spreading out of waves that are emitted or reflected by a moving source

what is red shift?

light source moves away from us, causing the wavelength of light reaching us to become longer

this shifts the light towards the red end of the em spectrum

(the light received from the star is redder than the light emitted by the star)

what is blue shift?

object moving towards earth causes the wavelength of light that reaches us to be shorter, shifting the light towards the blue end of the em spectrum

how does red/blue shift change with velocity

higher velocity => greater shift

what is the formula for red shift?

z=-v/c if v<0 so redshift is negative

why is the red shift formula only valid for v<

formula does not take into account the relativistic effects that may occur as object moves closer to the speed of light

how can you absorption lines to find the redshift of a star?

look at wavelength/frequency of absorption lines (e.g. Balmer series) in the observed spectrum and compare therm to the wavelength/frequencies that they should be

Z=-Δf/f

Z=Δλ/λ

where Δf is the difference between the observed and emitted frequencies (f observed - f emitted) if f>f observed star is red shifted and vice versa

what is cosmological red shift?

cosmological red shift is the idea that it is not galaxies that are moving away from us, but space itself is expanding and therefore stretching light waves

why is redshift useful to measure?

can be used as evidence that the universe is expanding at an accelerating rate (more distant an object, the greater its redshift)

what is the cosmological principle?

on a large scale the universe is`:

- homogenous (every part is the same)

- isotropic (everything looks the same in every direction - no centre)

what is Hubble's law?

the faster a galaxy is moving, the farther away it is

what did Hubble discover?

- spectra from galaxies all show red shift

- this can be used to find the recessional velocity of the galaxies

- Hubble plotted a graph of recessional velocity against distance, showing that they were proportional, suggesting that the universe is expanding

- Hubble constant = gradient of the graph

what is Hubble's constant?

the current expansion rate of the universe

why do the values of H have a large range?

distance is difficult to measure => H ranges from 50-100 kms-1 Mpc-1 but is approximately 65-80 kms-1 Mpc-1

why is Hubble's constant useful?

- shows that the universe is expanding

- can be used to calculate the age of the universe (1/H in SI units)

what is the Big Bang Theory?

- the idea that the universe started as an infinitely hot, dense singularity that exploded and has been expanding ever since

- at first there was high energy radiation everywhere from the explosion, but this has since cooled and redshifted to become the CMBR

what is the evidence for the big bang theory?

- redshift => acceleration of the universe

- CMB

- abundance of elements (H and He)

how can we estimate the age of the universe?

time=1/H

given that the universe has been expanding for the same rate for its whole life

what is there a limit on the size of the observable universe and why does it exist?

sphere with radius equal to the maximum distance light can travel during its age, taking into account the expansion of space

46-47 billion light years ≈ size of the observable universe

what is dark energy?

a type of unknown energy that fills the whole of space and may be the cause for the increasing rate of expansion of the universe

what is the cause for the universe expanding at an increasing rate?

dark energy

why is it though that the universe was decelerating until ≈ 5 billion years ago?

- all mass is attracted together by gravity

- attraction slows rate of expansion

what is cosmic microwave background radiation (CMBR) and how does it provide evidence for the model of the universe?

- what remains of em radiation produced in the early universe (evidence for big bang)

- microwave radiation that can be detected from all directions in space

-universe has expanded, which is why radiation is now in the microwave region

- has a perfect black body spectrum corresponding to a temperature of 2.73K

- homogenous + isotropic (agrees with cosmological principle)

- shows Doppler effect

how does of the abundance of H and He atoms provide evidence of the Big Bang Theory?

- nuclear fusion in the Big Bang converted hydrogen nuclei into helium nuclei

- this only lasted for a short period of time until the universe cooled too much + nuclear fusion stopped

- 1/4 of all H was fused into He => relative abundance of 3:1

- relative abundance today is ≈ 73% H, 25% He and 2% everything else

what are eclipsing binaries?

a pair of stars where the orbit of the stars is in the line of sight from earth to the system, meaning that the stars cross in front of each other as they orbit, giving them a characteristic light curve

what is a binary system?

two stars orbiting a close centre of mass

what are spectroscopic binaries?

binary systems in which the stars are too close to be resolved, so they must be identified using Doppler shifts

explain how doppler shifts are used to identify spectroscopic binaries

- when stars eclipse each other they are travelling perpendicular to the line of site => no doppler shift in emitted radiation

- when one star is travelling away from the observer and one is travelling towards, each spectral line is split in two => one is blue-shifted and the other is red-shifted

describe how you can use a graph to determine the orbital period of spectroscopic stars

- plot a light curve of apparent magnitude against time for an eclipsing binary system

- apparent magnitude drops when stars eclipse each other (drops more as dimmer star passes in front of brighter star)

- orbital period is the time taken for the cycle to repeat

what is a quasar?

objects with very large red shifts (so very far away) but also very bright absolute magnitudes

what is the power of a quasar comparatively?

same power as a galaxy (according to inverse square law)

how does a quasar form?

an active galactic nucleus (supermassive black hole) surrounded by a disc of matter emits jets of radiation as matter falls into the black hole

describe the 4 characteristics of a quasar

- extremely large optical red shift

- very powerful light output

- size not much bigger than a star

- emit Hydrogen Balmer lines of Hydrogen redshifted hugely

how can we estimate the power of quasars?

use inverse square law

brightness x 4πdistance^2 = power

distance is found using amount of doppler shift

brightness is radio not visible light

what is an exoplanet?

a planet that is not within our solar system (orbit other stars)

why are exoplanets difficult to detect?

- tend to be obscured by their host stars

- subtended angle is too small for most telescopes to resolve

explain how the Doppler shift/ radial velocity method can be used to detect exoplanets

- an exoplanet orbiting a star has a small effect on the stars orbit (causes small 'wobbles' in the orbit) => causes a Doppler shift in light received from the star

- this is because the star and exoplanet are orbiting around the centre of mass between them, but the star is much bigger so the COM is much closer to it

- wobble causes tiny red and blue shifts in the star's emissions, which can be detected on earth and suggest the presence of an exoplanet

- line spectrum is blue shifted when planet moves toward earth and red shifted when planet moves away from earth

- can calculate minimum mass of the exoplanet

- orbital period is equal to time period of Doppler shift

what are the problems with the radial velocity method?

- not enough of a gravitational pull on the star from low mass planets to cause a considerable effect on the orbit of the star => smaller change detected

- movement needs to be aligned withe the observer's line of sight for there to be a detectable shift in the light from the star

explain how the transit method is used to detect exoplanets

- if an exoplanet orbits a star, it passes in front of the star, blocking some of the star's light and causing a change in apparent magnitude

- change can be viewed as a dip in star's light curve

- regular dipping suggests a planet orbiting the star

- variation in the regularity of dips suggests several planets orbiting the star

- amount light curve dips by is dependent on the relative sizes of the star and planet (bigger planet relative to star causes a bigger dip in apparent magnitude) => can be used to find the radius of the planet (given radius of star)

what are the problems with using the transit method to detect exoplanets?

- planet's orbit must be perfectly aligned so that is passes directly between the star and the observer

- transit is likely to last only a very small fraction of the orbital period (especially if orbit is large), so it easy to miss (requires constant observation)

- can therefore only be used to confirm already observed exoplanets

What is the primary function of a lens in a telescope

to form an image by refracting light

what are the two main type of lenses

convex (converging) lenses and concave (diverging) lenses

what type of lens is primarily used in astrophysics module

convex (converging)

In a convex lens, where are parallel rays brought to a focus

at the focal point, along the principal axis

draw the lens diagram for an astronomical telescope

what is the equation for angular magnification (M)

M = (angle at eye) / (angle at unaided eye)

diagram of a cassegrain reflecting telescope

why do concave lenses always form virtual images

because it diverges light rays, making them appear to originate from a virtual focal point behind the lens. The image is always

- virtual

- upright

- smaller than the object

define focal point and focal length

Focal point - parallel rays converge after passing through a convex lens

focal length (f) - the distance between the optical centre of the lens and focal point

how dos a convex lens affect parallel incoming light rays

it converges them at a single point (focal point)

what is the purpose of the objective lens in a refracting telescope

to collect light from a distant object and form a real image

what role does the eyepiece lens play in a refracting telescope

it magnifies the real image formed by the objective lens

in a ray diagram for a refracting telescope, how are the objective and eyepiece lenses positioned relative to each other

they are aligned along the principal axis with the eyepiece positioned so that its focal point coincides with focal point of the objective lens

what is meant by normal adjustment

occurs when the focal points of eyepiece and objective lens coincide, ensuring that the emerging rays from eyepiece are parallel

why are converging lenses preferred in refracting telescopes

because they can focus parallel light rays from distant objects to a single point forming a clear image

what is chromatic aberration and how does it affect telescope observations

chromatic aberration is a distortion caused by a lens refracting different wavelengths of light by varying amounts

how does chromatic aberration affect telescope observations?

- leads to coloured fringes around the images

- only occurs in refracting telescopes

why does chromatic aberration happen?

because blue light has a shorter wavelength than red light meaning that it is refracted more by a lens than red light is because blue light has a bigger refractive index

how can chromatic aberration be minimized in telescopes

by using a second diverging lens which refracts light in the opposite direction.

allows red light to be brought to the same focal point as blue light.

what is spherical aberration

it occurs when light rays passing through the edges of a lens ae focused at different points than those passing through centre resulting in a blurred image

how can we reduce spherical aberration

it is reduced by using parabolic lenses (for refracting telescope)

a parabolic mirror (for a reflecting telescope)

what is the advantage of using a reflecting telescope over a refracting telescope

- a reflecting telescope avoids chromatic aberration and can be constructed with larger apertures, allowing for the collection of more light

- easier and cheaper to manufacture

- more compact design

what is meant by aperture

is the diameter of the objective lens/mirror, determining the amount of light the telescope can gather

why is a larger aperture beneficial

it allows for the collection of more light, resulting in brighter more detailed images

what is refraction and how does it relate to lenses

is the bending of light as it passes from one medium to another due to a change in speed

Lenses use refraction to focus or diverge light rays, forming images

how do convex and concave mirrors differ

convex (converging)- bends light inward to a focal point. used in telescopes

concave (diverging)- spreads light outward, used in correcting short sightedness

why are reflecting telescopes preferred

- no chromatic aberration (mirrors don't disperse light)

- Large mirrors are easier to manufacture than large lenses

- mirrors can be supported from behind

what advantages do space telescopes have e.g. Hubble over ground-based telescopes

- no atmospheric distortion (clearer images)

- Can observe across multiple wavelengths (e.g. infrared,UV,X-RAYS)

- constant conditions ( no weather or light pollution)

what is are the disadvantages of refracting telescopes

- chromatic aberration reduces image clarity

- expensive and difficult to manufacture large lenses without defects

- long heavy tubes are required for large apertures, making them less practical than reflecting telescopes

What is a Cassegrain telescope?

a reflecting telescope that uses a concave primary mirror and a convex secondary mirror to reflect light back through a hole in the primary mirror to an eyepiece

what is the primary structural difference between refracting and reflecting telescopes

a refracting telescope uses lenses to gather and focus light while reflecting telescopes use mirrors for the same purpose

what is the definition of minimum angular resolution of a telescope

the minimum angular resolution of a telescope is the smallest angular separation between two objects at which they can still be distinguished as separate entities

what role does a circular aperture play in telescopes

allows a cone of light to enter, focusing it into a region behind the aperture where it behaves as a point source

why does the human eye struggle to resolve two close objects?

the human eye has limited angular resolution, meaning objects closer than its resolving limit will appear blurred together due to diffraction effecrs

why do two point sources sometimes appear as a single source

if two sources are close together or viewed from a large distance, their diffraction patterns overlap, making them hard to tell they are separate sources

What is resolving power

Ability to distinguish between two closely spaced objects and produce them as separate images

what factors affect resolving power

- wavelength of light shorter wavelength yield better resolution

- aperture diameter- larger apertures provide higher resolving power

what does Rayleigh's criterion state

Two sources will be resolved if the central maximum of one diffraction pattern coincides with the first minimum of the other

what is collecting power

is a measure of the amount of light energy a telescope collects per second, determining the brightness of the image it produces

how does the collecting power of a telescope depend on its aperture

the collecting power is directly proportional to the square of the diameter of the objective lens/mirror

why does a larger aperture increase the collecting power of a telescope

a larger aperture increases the surface area available to collect light, allowing more photons to be gathered producing a brighter image.

How does a higher collecting power help in observing distant objects?

a higher collecting power allows a telescope to detect fainter objects at greater distances because more light is gathered, increasing image brightness

what are two advantages of a telescope with a larger aperture

Greater collecting power = brighter images

Greater resolving power = sharper images

how does the resolving power of a telescope depend on its aperture

the resolving power is inversely proportional to the apertures diameter

meaning larger apertures result in better resolutions

what are non optical telescopes

detect wavelengths outside the visible spectrum, including radio, infrared, ultraviolet, X-ray, and gamma-ray telescopes

why are non-optical telescopes used in astronomy and why are they important

different astronomical objects emit radiation at different wavelengths, revealing unique features not visible in optical telescopes

how does the earths atmosphere affect telescope observations

the atmosphere absorbs certain wavelengths, limiting what ground based telescopes can observe

completely absorbed - Gamma Rays, X-Rays , most UV, most IR

partially absorbed- some IR and UV wavelengths

most transparent- Visible and radio waves.

what are the advantages of space based telescopes

space telescopes avoid atmospheric interreference such as

-absorption of EM waves

-Light pollution from cities

-Distortion from atmospheric turbulence (e.g., twinkling stars)

what are the main components of a radio telescope

large parabolic dish (reflector) to collect radio waves

antenna & receiver to detect and amplify signals

what is meant by a light year

the distance travelled by light in a year

what is meant by a parsec

the distance from the centre of the earth to the centre of the sun

what is meant by the apparent magnitude

how bright something is from Earth

what is meant by absolute magnitude

how bright a star would be from 10 parsecs away

hipparcos scale

smaller apparent magnitude means the star is brighter

larger apparent magnitude means the star is dimmer

what is meant by a black body radiator

absorbs all incident wavelengths and therefore radiates at all wavelengths

order of classification of stars from hottest to coolest

O, B, A, F, G, K, M

colours of classification of stars

Blue - O, B, A

White - F

Yellow - G

Orange - K

Red - M

temperatures of classification of stars (in K)

25000+ - O

25000-11000 - B

11000-7500 - A

7500-6000 - F

6000-5000 - G

5000-3500 - K

3500 and below - M

draw a hertsprung-russel diagram